This invention relates to articles, such as a coated building panel, that contain calcium sulfate hemihydrate. More specifically, it relates to a plaster-like material for use in coatings and articles that has the feel of plaster during application but does not hydrate.
Gypsum is a common material that is widely used in the building industry for a number of uses. The usefulness of this material is due, at least in part, to its reasonable cost and the ability to form it into substantially any shape. Calcium sulfate hemihydrate, also known as stucco or calcined gypsum, is mixed with water to form a paste or slurry. The slurry is shaped by any known means, such as, but limited to, casting it into a mold, spraying it, pouring it onto a surface or applying it with a trowel. The calcium sulfate hemihydrate reacts with the water in a hydration reaction to form an interlocking matrix of set calcium sulfate dihydrate (gypsum) crystals. Although some shrinkage often occurs, the gypsum generally retains the shape into which it was formed prior to completion of the setting reaction.
Interior walls are typically made of gypsum panels, such as SHEETROCK® brand gypsum panels, cement panels or the like. While the use of panels allows large areas of wall to be covered quickly, seams between the panels must be finished to make a monolithic surface that is ready to receive paint or another decorative finish.
Finishing of the panels is usually a time consuming process. After the boards are hung, a coat of joint compound is applied to the seams and the nail heads. As it dries, the joint compound shrinks, leaving a depression in the area of the seam that is the last to dry. The joint compound is allowed to dry overnight, then is sanded when it is completely dry. Dust from sanding is removed with a tack cloth or damp sponge. A second coat of joint compound is applied and allowed to dry overnight followed by sanding and dedusting of the surface. For a quality job, the process is repeated with a third coat of joint compound. Finishing of the surface takes at least three days, even after the boards are hung. This process can also be used to repair damaged walls.
Although this process takes time, it produces a smooth, monolithic surface at a relatively low cost. It is a simple process and can often be done by a homeowner without professional assistance. But there are drawbacks in the amount of time needed to complete such a job. In new construction, installation of cabinets, carpet, fixtures and other finishing items is held up while waiting for the finishing to be completed. Workmen have to keep returning to the same job site several days in a row. For a homeowner doing repairs or redecorating, disruption of the home is prolonged. Repeated sanding of the surface generates fine dust that permeates closets and cabinets, requiring a thorough dusting throughout the home. A quicker and cleaner method of finishing walls would be desired by both contractors and homeowners.
An alternative method of finishing interior walls and ceilings includes applying a veneer or “skim” coat of plaster over the entire gypsum panel surface. This technique results in a stronger surface that is less likely to chip, dent, scratch or incur nail pops than finishing using a joint compound. The surface needs no additional preparation. Use of colorized plaster can even eliminate the need for painting of the plastered surface. A single coat of plaster over the entire wall surface is faster than applying multiple coats of a joint compound when drying time is considered.
The availability of a ready-mixed plaster composition would be desirable for a number of applications. Such compositions could be used, for example, for finishing walls, dental applications, casts, moldings, machinable articles, sprayable plasters, and other applications that will be known by one skilled in the art.
In U.S. Pat. No. 4,661,161, a ready-mixed, setting-type joint compounds was formulated using calcium sulfate hemihydrate as the binder. A set retarder was added to the wet paste to stop the hemihydrate from hydrating to the dihydrate form. Hydration was intended to be initiated by the addition of a set accelerator just prior to use of the joint compound. However, it was found that the set retarder degraded over time, allowing hydration to occur during the shelf-life of the product.
A ready-mixed, setting-type joint compound is disclosed in U.S. Pat. No. 5,746,822. This composition uses phosphates, such as tetra sodium pyrophosphate for a long-lasting set retardation and accompanying long shelf like. However, these joint compounds were found to be unsatisfactory for many applications as they had a relatively low strength.
U.S. Pat. No. 5,779,786 reveals a ready-mixed, setting-type joint compound using acrylic acid and acrylamide repeating units. These formulations have reduced water bleed and sedimentation than other joint compounds, however they are also low in strength, and useful only in certain applications.
The compositions above are intended for use as joint compounds and are formulated to that use. However, a typical joint compound does not have the same “feel” as a plaster composition as it is being applied. A skilled plasterer knows exactly how much pressure to apply to the trowel to get the correct thickness when applying a skim or veneer coating of plaster to a wall. Use of a conventional ready-mixed joint compound by a plasterer would require learning the feel of the joint compound in order to apply it correctly. More importantly, joint compound skim-coating is labor and time intensive, requiring application of the compound, drying time and sanding of the entire surface, which also creates a large amount of dust. On the other hand, veneer plastering only requires application and densification of the plaster to attain a superior smooth monolithic surface.
Thus there is a need in the art for a composition that has the same working properties as plaster to facilitate application to a wall, such as plaster feel, no sanding, and fewer application steps.